Glycine max seeds are normally inoculated with Bradyrhizobium japonicum before planting, but if I am attempting to see differences in nodulation between treatments with addition of a potentially toxic material I would rather inoculate the soil in pot experiments, because the bacterium is not native in my area and thus would be unlikely to find. The point is to draw a parallel with areas that has previous history of G. max growth and would not necessarily need seed inoculation because B. japonicum would already be present in the soil. Or would it be irrelevant to think this way, because a lot of farmers inoculates every growth season?
I think seed inoculation is the best way to enrich specific rhizobium provided you have enough population ( 108-1010 per mL).
I think it is valid, and important to evaluate the effect of the external variable in (1) seed-inoculation and (2) soil-enriched conditions. The degree of bacteria and rhizosphere proximity is different and, perhaps, in case 2, there is more effect of the external variable on plant-bacteria communication.
In our area farmers do not inoculate every year, but every other or third season. There is companies trying to find persevering strains, but currently they are lost within +/- three years.
In Brazil, soybean farmers inoculate every year. Even in soils with established Bradyrhizobium populations, inoculation promotes an increase of 8% in grain yield.
Thank you for answers. An additional question: since the nodules usually form around V2 stage, would it be possible to germinate seedlings without rhizobia inoculation and then transplant them to pots with the external variable and the inoculated soil at around that stage? To ensure all plants established nodulation with the same start point and all had the same starting conditions.
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